An oxidized catalyst precursor for forming highly active and selective catalysts useful for conducting hydrogenation reactions, particularly carbon monoxide hydrogenation reactions, especially Fischer-Tropsch reactions; and the catalyst produced from the oxidized catalyst precursor.
Processes for the hydrogenation of carbon monoxide to produce waxy and/or oxygenated products for upgrading to highly valued chemical raw materials and/or hydrocarbon fuels are well documented in the technical and patent literature. For example, in the Fischer-Tropsch (F-T) process, it is well known that the carbon monoxide component of synthesis gas can be catalytically converted by reaction with the hydrogen to reduction products constituting a range of waxy liquid hydrocarbons; hydrocarbons which can be readily upgraded to transportation fuels. In these processes, e.g., catalysts constituted of Group VIII metals (Periodic Table of the Elements, Sargent-Welch Scientific Company, Copyright 1968), notably the Iron Group metals, particularly iron, ruthenium and cobalt, are generally preferred for the synthesis of C5+ hydrocarbons; and copper has become the catalytic metal of choice for alcohol synthesis. These metals can exist in multiple valence states in the catalyst precursors and each state can display quite different behavior from the others during the reduction treatments, hence impacting the catalytic properties of the active catalyst. Each of the metals can be promoted or modified with an additional metal, or metals, or oxide thereof, to improve, e.g., the activity and/or selectivity of the catalyst in conducting these reactions.
It is known that Iron Group metal surfaces exhibit higher activities for catalytic reactions such as hydrogenation, methanation and F-T synthesis when catalysts on which these metals are dispersed are subjected to high temperature oxidation, and subsequent reduction. Recent art can be found in Applied Catalysis, A, General 175 (1998) pp 113-120 and references therein. U.S. Pat. Nos. 4,492,774; 4,399,234; 4,493,905; 4,585,789; 4,088,671; 4,605,679; 4,670,414 and EPO 253924 disclose activation of cobalt catalysts by means of a reduction/oxidation/reduction (R-O-R) cycle, resulting in an increase in activity for F-T synthesis. Thus, typically such catalyst, e.g., supported reduced Co, is contacted at high temperature ranging from about 300xc2x0 C. to about 600xc2x0 C. with a gaseous oxygen-containing stream to oxidize the metal, or metals, to its most stable oxide form, e.g., Co3O4. Precautions are taken during such treatments to control the exothermicity of the reaction to avoid sintering of the oxide metal particles, which can be detrimental to the activity of the catalyst. On reduction, i.e., on completion of the oxidation-reduction cycle, the dispersed oxide particles, (e.g., the Co3O4) are reduced to dispersed metal particles and the catalytic activity is increased.
Considerable progress has been made in the development of catalysts, and processes, these providing good activity, and selectivity in alcohol synthesis, and in the conversion of hydrogen and carbon monoxide to distillate fuels, predominantly C5+ linear paraffins and olefins, with low concentrations of oxygenates. Nonetheless, there remains a pressing need for improved catalysts, and processes; particularly more active catalysts, and processes, for producing transportation fuels and lubricants of high quality at good selectivity at high levels of productivity.
This and other needs are achieved in accordance with the present invention embodying a low temperature oxidation process which requires, beginning with a catalyst precursor constituted of a composite of a solids support, or powder, and a compound or salt of a metal, or metals, inclusive of cobalt, catalytically active for conducting hydrogenation reactions, notably hydrogenation reactions, contacting the catalyst precursor with liquid water or steam, or a mixture of liquid water and steam, at sufficiently low temperature to oxidize and convert at least a portion of the metal, or metals component of the catalyst precursor to a metal hydroxide, low oxygen-containing metal oxide, or mixture of metal hydroxide and low oxygen-containing metal oxide. By oxidation is meant the conversion of a metal species to a low valence state, e.g., Co, the Co species, to a Co2+ species. For example, in a low temperature oxidation treatment of a cobalt/TiO2 catalyst precursor treated with liquid water or steam, or a mixture of liquid water and steam, all or a portion of the cobalt component of the catalyst precursor is oxidized and converted to Co2+, i.e., a hydroxide of cobalt, Co(OH)2, low oxide of cobalt, CoO, or mixture of these components; these components becoming intimately contacted with the surface of the support, or powder. On reduction, as may be produced by contact and treatment of the oxidized catalyst precursor with hydrogen, the dispersed metal oxide or hydroxylated catalytic metal, or metals component of the catalyst, e.g., CoO or Co(OH)2, or mixture thereof, is reduced to elemental or metallic metal, e.g., Co; and the catalyst thereby activated. Optionally, the oxidized catalyst precursor may be dried in a non-oxidizing atmosphere and the hydroxide converted to a low oxygen content oxide, i.e., CoO. Optionally also, the oxidized catalyst precursor may be dried and calcined at elevated temperature in an oxidizing atmosphere to obtain a metal oxide or metal oxides, e.g., Co3O4. In both options, the catalyst is activated by reduction of the oxidized catalyst precursor. The oxidized catalyst precursor, and catalyst made therefrom are useful compositions of matter, the activated catalyst being particularly useful for efficiently conducting hydrogenation reactions, notably carbon monoxide hydrogenation, especially F-T synthesis reactions, to provide a variety of useful products.
The composition, comprising the support and catalytic metal, or metals component, on contact with the water or steam, or mixed phase water and steam, at low temperature is transformed: the catalytic metal(s), component of the catalyst, e.g., Co, is oxidized and converted into metal hydroxides, low oxygen-containing metal oxides, or metal hydroxides admixed with oxides of the metal in low valence state, e.g., CoO, Co(OH)2. It is found that the transformed metal, or metals, e.g., CoO or Co(OH)2, is more readily, widely and intimately dispersed on the surface of the support than a higher valence more stable oxide form, e.g., Co3O4; providing on reduction smaller crystallites of the metal, or metals which are a more highly active species than is produced by reducing Co3O4 to form the catalyst. The greater activity and stability of catalysts made by this process, and the fact that the oxidation step can be carried out at low temperature in an aqueous medium, or by simple contact with liquid water, or steam, or mixed phase of water and steam, are consequences of considerable import in the development of an F-T process.
The catalytic metal(s) of the catalyst precursor, on contact with the oxidizing liquid water or steam, or mixture thereof, converts at low temperature to its hydroxide or low valence oxide. Reactions taking place in this conversion for a cobalt based catalyst precursor thus include the following:
Co+H2O less than = greater than CoO+H2xe2x80x83xe2x80x83(1)
CoO+H2O less than = greater than xe2x80x9cCo(OH)2xe2x80x9d,xe2x80x83xe2x80x83(2)
or the sum of reaction 1 and reaction 2:
Co+2H2O less than = greater than xe2x80x9cCo(OH)2xe2x80x9d+H2xe2x80x83xe2x80x83(3)
The hydroxide of cobalt is shown as xe2x80x9cCo(OH)2xe2x80x9d in the above equations since its exact form can be more complicated than the pure metal hydroxide because with the low temperature treatment with liquid water or steam, the hydroxide of cobalt that is formed can interact with the support material (e.g., the TiO2). The oxidation of the metal, as depicted in reactions 1 and 3, with liquid water or steam is considerable less exothermic than the oxidation of the metal directly with molecular oxygen. In addition, the exothermicity of the oxidation reaction is effectively moderated by the presence of excess water; especially liquid water. The metal hydroxide or oxy-anion(s) are intimately dispersed on the surface of the support, hence providing upon reduction with hydrogen or a hydrogen-containing gas small crystallites of the metal, or metals which are highly active species for carbon monoxide hydrogenation. Optionally, when the hydroxylated catalyst precursor is calcined, the metal hydroxide or low valence oxide particles are further oxidized to small oxide particles without the deleterious effect of the intense exothermic reaction of directly converting a reduced metal to the higher valence oxide, e.g., Co metal to Co3O4.
In the low temperature liquid water or steam oxidation treatment at least a portion of the catalytic metal component of the catalyst precursor is oxidized to lower valence metal hydroxide or oxide by contact with liquid water or steam at temperatures ranging from about 25xc2x0 C. to about 275xc2x0 C., preferably from about 100xc2x0 C. to about 250xc2x0 C., most preferably from about 150xc2x0 C. to about 225xc2x0 C., at not less than autogenous pressure, or pressures ranging from about 1 atmosphere (atm) to about 50 atm, preferably from about 1 atm to about 20 atm, for periods ranging from about 0.1 hour to about 24 hours, preferably from about 0.25 hour to about 10 hours; or until loss of pyrophoricity. In a preferred mode of practicing this invention the catalyst precursor is dispersed or slurried in the liquid water, e.g., by containment in a reaction vessel, or autoclave. The metal, or metals, component of the catalyst precursor treated in such manner is transformed at the low temperature into a low oxygen-containing metal oxide or metal hydroxide, or mixture thereof. As will be recognized, the contacting time will be sufficient as required to obtain the desired amount of oxidation. Depending upon the design of the process, e.g., fixed bed, slurry bubble, etc., the amount of water used varies greatly. For example, in a fixed bed operation, the water either in a liquid or steam phase or both is added in a flow-through mode. Typically, the fixed bed is fed water continuously which fills the void volume of the bed. This continuous feed of water has the added effect of sweeping out of the reactor gaseous reaction products (e.g., H2) and, thus, drives reactions 1 and 3 to the right as oxidized metal products. For an operation associated with the use of a bubble column or a moving bed, the water oxidation may be carried in a batch or continuous mode. Regardless of the method use, the weight of water to the weight of catalyst varies typically from about 1:5 to about 100:1, preferably from about 1:1 to about 50:1, and most preferably from about 2:1 to about 10:1.
In the low temperature water or steam treatment, a significant portion of the catalytic metal component of the catalyst precursor is thus oxidized to metal hydroxides or lower metal oxides, whereas high temperature oxidation with a molecular oxygen containing non-hydrated gas stream as described in the prior art, produces essentially complete oxidation of the metals component to the most stable oxide phase. For example, in the treatment of the catalyst precursor with liquid water or steam, or mixture thereof, the Co metal species is oxidized to CoO, Co(OH)2, or both CoO and Co(OH)2 rather than Co3O4. The catalyst precursor composition containing the Co2+ metal oxidized species provides significantly different behavior from the composition obtained by conventional high temperature oxidation of the catalyst precursor with the non-hydrated oxygen containing gas.
The catalyst precursor subjected to the low temperature oxidation treatment and used in accordance with this invention is characterized as the composite of a solid support, or powder component and a compound or salt of a catalytic metal, or metals component, inclusive of cobalt, formed by gellation, cogellation or impregnation techniques; e.g., precipitation of gels and cogels by the addition of a compound, or compounds of the catalytic metal, or metals, from solution as by addition of a base, or by the impregnation of a particulate solids support, or powder, with a solution containing a compound or salt of the catalytic metal, or metals; techniques well known to those skilled in this art. The catalytic precursor within the meaning of this invention is thus the harbinger composition which, when the compound(s) or salt(s) of the catalytic metal, or metals, inclusive of cobalt, is oxidized, by contact with water or steam, or mixture of water and steam and then reduced, as by contact with hydrogen, is comprised of sufficient of the dispersed reduced catalytic metal, or metals, that it is useful in catalyzing hydrogenation reactions. In such preparation procedures a metal, or metals, inclusive of cobalt, catalytically active for conducting hydrogenation reactions, is composited with a particulate solids support, or powder, suitably a refractory inorganic oxide support, preferably a crystalline aluminosilicate zeolite, natural or synthetic, alumina, silica, silica-alumina, titania, zirconia or zirconia-silicates, or mixtures of these oxides, or the like. For example, in impregnating a particulate support, or powder, the support or powder is contacted with a solution containing a salt, or compound of cobalt; and if desired, an additional metal, or metals, preferably a Group VIIB or Group VIII metal, or metals, of the Periodic Table of the Elements, or copper or thorium can be used to further modify or promote the catalytic reaction. Generally, from about 2 percent to about 70 percent, preferably from about 5 percent to about 50 percent metallic metal, or metals, inclusive of cobalt, is deposited upon the particulate solids support or powder, based upon the total weight (wt. %; dry basis) of the catalyst precursor (or the finished catalyst). Catalysts having a relatively high metal, or metals, loading are preferred because these catalysts can be loaded into slurry bubble columns over a broad range of concentrations for activation, and use for conducting F-T reactions up to that high concentration in which mixing and pumping the slurry becomes limiting. The impregnated powder or support may then be contacted with a reducing agent, suitably hydrogen at elevated temperature, to reduce the metal component to its low valence state, generally to metallic metal.
In conducting the low temperature oxidation treatment of a catalyst precursor, a preferred procedure is generally as follows:
The catalyst precursor is slurried in liquid water; the slurry of water:catalyst precursor being contained in the reactor in volume ratio of at least about 0.5:1, preferably at least about 2:1, and higher. A temperature ranging from about 25xc2x0 C. to about 275xc2x0 C., preferably from about 100xc2x0 C. to about 250xc2x0 C., most preferably from about 150xc2x0 C. to about 225xc2x0 C., and a total reaction pressure ranging from about 1 atm to about 50 atm, preferably from about 1 atm to about 20 atm is maintained. The contact time between the catalyst precursor, or deactivated catalyst and the water ranges generally from about 0.01 hour to about 40 hours, more preferably from about 0.1 to about 10 hours, and most preferably ranges from about 0.2 hours to about 2 hours, or up to the point in time where the oxidized catalyst precursor, or catalyst, loses its pyrophoricity.
The catalyst precursor in the slurry is next separated from the water by evaporating or by filtering off the excess water and drying, and may then be further treated, or reduced as by contact with hydrogen, or a hydrogen-containing gas, at elevated temperature, preferably at temperature ranging from about 200xc2x0 C. to about 600xc2x0 C., preferably from about 300xc2x0 C. to about 450xc2x0 C., at hydrogen partial pressures ranging from about 0.1 atm to about 100 atm, preferably from about 1 atm to about 40 atm, sufficient to convert the metal hydroxide, low oxygen-containing metal oxide, or mixture thereof to essentially the zero valent state, i.e., metallic metal.
The catalysts, or oxidized catalyst precursors after they have been reduced are used in a hydrogenation process, preferably a carbon monoxide hydrogenation process, particularly one wherein liquid, gaseous or solid hydrocarbon products are formed by contacting a synthesis gas comprising a mixture of H2 and CO with the F-T hydrocarbon conversion catalyst of this invention under water gas shifting or non-shifting conditions; but preferably non-shifting conditions in which little or no water gas shift reaction occurs, particularly when the catalytic metals comprise Co and Re or Ru or mixtures of one or both of these latter metals with cobalt.